The present invention relates to an idling control system for an automatic transmission, and more particularly relates to such an idling control system for an automatic transmission which provides improved anti creep characteristics for keeping the vehicle at rest and for starting off the vehicle from rest and particularly has a smooth starting off action and is substantially immune from undesirable performance variations due to constructional variations within manufacturing tolerances.
The present patent application has been at least partly prepared from material included in Japanese Patent Applications Ser. Nos. 60-117559, 61-083748, and 61-083749, which were filed by an applicant the same as the assignee or entity owed duty of assignment of the present patent application, and the present application hereby incorporates into itself by reference the texts of said Japanese Patent Applications and the claims and the drawings thereof; copies are appended to the present application.
There is a conventional type of automatic transmission for a vehicle which comprises a fluid torque converter which receives output of rotational power from an internal combustion engine and which transmits said power with a certain degree of torque amplification and with a certain degree of slippage to a gear transmission mechanism which can selectively provide any one of a plurality of forward speed stages (gearing ratios) between its power input shaft thus connected to said torque converter and its power output shaft connected to the driven road wheels of the vehicle so as to transmit rotational power thereto. Such a torque converter provides an amount of torque amplification and an amount of slippage both of which vary as the rotational speed of the input member (or pump impeller) of said torque converter varies, according to determinate performance characteristics thereof. And the gear transmission mechanism, which typically comprises a plurality of selectively engagable gear trains, is set in a determinate way to one or the other of said plurality of forward speed stages, according to the current values of various operational parameters of the internal combustion engine and of the vehicle incorporating it, said operational parameters including but not being limited to engine load (usually taken as being represented by throttle opening amount), vehicle road speed, and the one of various shift ranges such as "D" or drive range, "S" or second range, and "L" or low range currently selected on a manually controlled range selection means provided to the vehicle. And the manner in which said plurality of forward speed stages is thus selected between according to the current values of said vehicle operational parameters is conventionally described by one or more shift diagrams, typically one for each one of such forward shift ranges which encompasses more than one speed stage.
When the abovementioned manual range setting means is set to "D" or drive range or some other forward driving range and the vehicle speed is substantially zero, i.e. the vehicle is substantially at rest, and further the internal combustion engine is substantially idling: then, in view of the desirability of maintaining a capability for relatively rapid starting off of the vehicle from this stopped condition, it is usually practiced not to put the gear transmission mechanism into a neutral condition in which it is incapable of transmitting rotational power between its power input shaft and its power output shaft, but instead typically said gear transmission mechanism is left as engaged to its first speed stage, i.e. its speed stage in which it provides a maximum reduction gearing ratio between its power input shaft and its power output shaft. Thereby a rapid response is available when the accelerator pedal of the vehicle is pressed in order to move off from rest, and the transmission is maintained in a state ready to transmit rotational power from the engine to the wheels of the vehicle, with the torque converter providing relatively great slippage and transmitting a relatively small amount of torque at this time due to the relatively low rotational speed of its power input member which is being driven by the internal combustion engine which is idling. When the accelerator pedal is in fact thus depressed by the foot of the vehicle driver, the rotational speed of the power output member of the engine (typically its crankshaft) rises, and accordingly the rotational speed of the power input member of the torque converter rises, and thereby the torque converter is caused to transmit substantially more torque, i.e. to provide less slippage between its power input member and its power output member. This increased torque is immediately transmitted by the gear transmission mechanism, since it is being maintained as engaged to its first speed stage as explained above, to the driven road wheels of the vehicle; and thereby the vehicle is caused to move off from rest.
This leaving of the gear transmission mechanism engaged to its first speed stage during the engine idling vehicle stopped condition produces the consequence of being liable to still transmit some substantial amount of torque from the torque converter through said gear transmission mechanism to the driven road wheels of the vehicle at this time. If not counteracted, this produces so called creep of the vehicle, and indirectly leads to engine idling vibration. Further, it becomes necessary for the driver of the vehicle to press relatively hard on the brakes of the vehicle while the vehicle is thus stopped, as for example at a traffic light or the like, in order positively to prevent forward motion of the vehicle. And the drag of the torque converter on the internal combustion engine causes the fuel economy of the engine to be bad, and furthermore the transmission fluid in said torque converter tends to become hot, which can in some cases lead to an overall overheating problem for the vehicle as a whole.
This type of gear transmission mechanism typically includes a clutch which is generally called a forward clutch which is engaged when and only when the gear transmission mechanism is engaged to one of its forward speed stages, i.e. is providing forward driving to the vehicle. This forward clutch typically comprises two sets of friction elements connected to and two members which it selectively engages together, and is typically actuated by said two sets of friction elements being pressed together by a hydraulic servo device which has a pressure chamber for its actuation. Thus, the forward clutch is engaged when hydraulic fluid pressure is supplied to said pressure chamber of said hydraulic servo device thereof, with the engagement pressure between said sets of friction elements of said clutch and accordingly its torque transmission capability increasing the greater is the pressure value of said hydraulic fluid pressure. Accordingly, the assignee of the present patent application, who is also the applicant of the Japanese patent application of which the priority is being claimed for the present application, has proposed--in Japanese Patent Applications Ser. Nos. 41-18128 (1966), 56-117742 (1981), 57-10444 (1982), 57-75829 (1982), 57-115087 (1982), and 59-176300 (1984), the first of which has been published as Japanese Patent Publication Ser. No. 47-19962 (1972), and the next listed ones of which have been respectively laid open as Japanese Patent Laying Open Publications Ser. Nos. 56-21047 (1981), 58-128552 (1983), 58-193953 (1983), and 59-6454 (1984), and none of which is it intended hereby to admit as prior art to the present patent application except to the extent otherwise required by applicable law--that, in the above described condition when the engine of the vehicle is substantially in the idling condition and the vehicle speed is substantially zero, the hydraulic fluid pressure, which is thus being supplied as an actuating pressure to a frictional engaging device of the gear transmission mechanism which is of the so called forward clutch type which is engaged when and only when the gear transmission mechanism is engaged to one of its forward speed stages, should be substantially decreased. Doing this allows said forward clutch to slip somewhat during this engine idling vehicle stopped operational condition, so that the gear transmission mechanism no longer transmits torque to any substantial extent while the vehicle is stopped. This means that substantially no vehicle creep takes place, and thus there is no requirement for the driver to press on the vehicle brake to maintain the vehicle in the stationary state. Further, the occurrence of idling vibration is made much less likely. The fuel economy of the vehicle is also improved. Also, since there is substantially less churning up of the transmission hydraulic fluid in the fluid torque converter, the problem of heating up of said torque converter is substantially overcome. The actuating pressure for the forward clutch can be quickly raised to an appropriate operational level when the accelerator pedal of the vehicle is depressed in order to start off the vehicle from rest, and so the provision of this anti creep device does not make the obtaining of a good response for starting off the vehicle much more difficult.
A problem that has occurred with such a system is as follows. In the above outlined construction, during the condition when the engine of the vehicle is substantially in the idling condition and the vehicle speed is substantially zero, the actuating hydraulic fluid pressure for the forward clutch is ideally reduced to a so called waiting value, which should be as high as possible short of causing said forward clutch to provide substantial torque transmission, so that said forward clutch is ready to be rapidly returned to the engaged state by moderate increase of said actuating pressure. However, this so called waiting value of actuating pressure is not defined with absolute accuracy when the transmission is being manufactured, i.e. is not always quite the same for each transmission unit of a series that are being made by mass production, and further does not always remain constant even for one transmission unit when operational conditions vary. This is because of the effect of inevitable variances from ideal specifications caused by manufacturing tolerances in, for example, the stroke of the piston of the hydraulic actuator for the forward clutch, and the strength of the return spring for said hydraulic actuator. Further, variation in the temperature of the transmission can cause these parameters to vary during operation, and can also cause fluctuations in the effectiveness of, for example, the sealing effectiveness of seal rings included in the hydraulic actuator. Because of all this, the ideal value of the waiting value for this forward clutch actuator hydraulic pressure varies. Since the prior arts outlined above have not been able to compensate for such variation, it has been required to set said forward clutch actuator waiting pressure to some compromise value, which typically has been on the low side, since error on the high side leads to undesirably high transmission creepage; but this has inevitably meant that in practice the desirable results obtained by the anti creep control system not setting the forward clutch waiting pressure to zero, particularly quick engagement when the vehicle is to be moved off from rest, have been largely lost.
Another problem that has arisen in this connection is that of preventing snatching of the transmission. If the forward clutch is not quickly enough engaged when anti creep action is to be terminated, then there is a risk that the torque converter revolution speed will suddenly rise up and then meet with a snatch, thus suffering an unpleasant shock. And the proper setting of the anti creep control system actually depends upon whether or not various engine ancillary devices, such as an air conditioner pump, an alternator, and the like, are operating or not, as well as upon engine warming up state and other engine operational parameters.